The present invention relates to a process for producing a polyarylene sulfide (hereinafter referred to as PAS) and, particularly, the present invention relates to a process for producing a medium- or high-molecular-weight PAS within a short time period without using a polymerization aid such as an organic acid salt.
More in detail, the present invention relates to (1) a process for producing a medium-molecular-weight PAS which comprises the steps of subjecting an alkali metal sulfide and a dihalo aromatic compound to dehalogenation and sulfidation reaction at a temperature of from 235 to 280.degree. C. in the presence of a water in amount of from 0.5 to 5 mol per 1 kg of an aprotic organic polar solvent until the conversion of the dihalo aromatic compound reaches from 70 to 98 mol %, thereby forming a PAS prepolymer having a melt viscosity of from 5 to 300 poise (as measured at 310.degree. C. under a shear rate of 200 sec.sup.-1), adding water to the resultant reaction mixture containing the PAS prepolymer so that from 6 to 15 mol of water is present per 1 kg of the solvent, and then continuing the reaction at a temperature of from 240.degree.-290.degree. C., in which the reaction comprising the said two steps is conducted for 1 to 10 hours, and (2) a process for producing a high-molecular-weight PAS which comprises the steps of subjecting an alkali metal sulfide, a dihalo aromatic compound and a tri- or higher polyhalo aromatic compound in an amount of from 0.01 to 1 mol % based on the dihalo aromatic compound to dehalogenation and sulfidation reaction at a temperature of from 235 to 280.degree. C. in the presence of a water in amount of from 0.5 to 5 mol per 1 kg of an aprotic organic polar solvent until the conversion of the dihalo aromatic compound reaches from 70 to 98 mol %, thereby forming a PAS prepolymer having a melt viscosity of from 5 to 300 poise (as measured at 310.degree. C. under the shear rate of 200 sec.sup.-1), adding water to the resultant reaction mixture containing the PAS prepolymer so that from 6 to 15 mol of water is present per 1 kg of the solvent, and then continuing the reaction at a temperature of from 240.degree.-290.degree. C., in which the reaction comprising the said two steps is conducted for 1 to 10 hours.
In recent years, thermoplastic resins of higher heat-resistance have been increasingly demanded as a material for the production of electronic equipments and automobile parts.
While PAS has properties capable of satisfying these requirements, it is difficult to obtain PAS typically represented by polyphenylene sulfide of sufficiently high molecular weight and accordingly, there has been a problem that it is very difficult to obtain fibers or films requiring high strength and molded products requiring high impact strength.
As a typical process for the production of PAS, a process of reacting a dihalo aromatic compound and sodium sulfide in an organic amide solvent such as N-methylpyrrolidone has been disclosed in Japanese Patent Publication No. 45-3368 U.S. Pat. No. 3,354,129). However, PAS produced by this proposed method is of a low molecular weight and it is difficult to mold it into molded articles, films, sheets, fiber, etc.
In view of the above situations, various methods of improving the process for producing medium- or high-molecular-weight PAS have been proposed. In the present specification, "low-molecular-weight PAS", "medium-molecular-weight PAS" and "high-molecular-weight PAS" mean those PASs having melt viscosity (as measured at 310.degree. C. under the shear rate of 200 sec.sup.-1) of not more than about 300 poise, about 300-1000 poise and not less than about 1000 poise respectively.
Among the improved methods, a most typical method as described in Japanese Patent Publication No. 52-12240 (U.S. Pat. No. 3,919,177) uses an alkali metal organic acid salt as a polymerization aid in the reaction system. According to this method, the polymerization aid has to be added approximately in an equimolar amount to the alkali metal sulfide and, further, it is required to use a large amount of lithium acetate or sodium benzoate which is expensive in order to obtain PAS of high polymerization degree, and accordingly the production cost of PAS is increased, resulting in an industrial disadvantage.
Further, according to this method, a large amount of organic acid, etc. are introduced into waste water upon recovery of PAS after the polymerization reaction, thereby possibly causing a problem in view of public pollution in this method and there are serious problems from the economical point of view such as requirement of enormous facilities and much running cost for the recovery and reuse of the organic acid, etc. for prevention of such problems.
As another method of producing medium- or high-molecular-weight PAS, there has been proposed a method of using a tri- or higher polyhalo aromatic compound as a crosslinking agent or branching agent during polymerization or at the terminal stage of polymerization [Japanese Patent Application Laid Open (KOKAI) No. 53-136100 (U.S. Pat. No. 4,116,947) etc.].
However, according to this method, although high-molecular-weight PAS having apparent melt viscosity of several tens of thousands poise can be obtained easily, since this PAS is a highly crosslinked or branched polymer, it is poor in the processability, particularly, stretching property and fiber-forming property, and it is difficult to mold into films or fibers. Further, even if molded articles are obtainable, they are mechanically extreme since the molecular chain is basically short.
In view of the foregoing problems, the present inventors have made an extensive study on the polymerization mechanism of the alkali metal sulfide and dihalo aromatic compound in a simple polymerization system in order to find a process for producing PAS having a sufficiently high melt viscosity, and excellent in the processability and the mechanical property at a reduced cost without using a polymerization aid such as an alkali metal organic acid salt and/or an alkali metal sulfonic acid salt, and as a result, it has been found that a linear PAS of remarkably high molecular weight can be produced without using any polymerization aid by making a significant difference in various polymerization conditions, particularly, the amount of coexistent water and the polymerization temperature between 1st polymerization step (preliminary polymerization step) and 2nd polymerization step (two-phase separated polymerization step) [Japanese Patent Application Laid Open (KOKAI) No. 61-7332 (U.S. Pat. No. 4,645,826)].
However, in the method of Japanese Patent Applicaton Laid Open (KOKAI) No. 61-7332, the total polymerization time is still as long as more than 10 hours and the productivity per unit volume of the polymerization reactor is somewhat lower as compared with other general synthetic resins. In view of the above circumstances, the present inventors have further examined the process disclosed in Japanese Patent Application Laid Open (KOKAI) No. Sho 61-7332 and made an intensive study on the process of shortening the polymerization time and simplifying the process in order to find a method capable of simply producing medium to high-molecular-weight PAS, particularly, medium-molecular-weight PAS in a remarkably shortened time period.
As a result, it has been found that the polymerization time required for obtaining medium to high-molecular-weight PAS can drastically be shortened in the process as described above, particularly, by increasing the polymerization reaction rate at the preliminary polymerization by elevating the reaction temperature, adding a large amount of water at a certain timing before the starting of abnormal reaction (decomposing reaction, etc), causing two-phase separation while elevating the reaction temperature, and continuing the polymerization reaction until the melt viscosity reaches a desired level, but stopping the reaction before the melt viscosity begins decreasing. That is, it has been found that it is important to solve the problem how to control the timing of adding water in the course of the polymerization reaction, and also found that high-molecular-weight PAS can be obtained predominantly by further adding a slight amount of a crosslinking agent, and on the basis of these findings the present invention has been attained.